Thank you for your question. The propagation loss can be found from the imaginary part of the effective index of the waveguide modes, and is provided automatically by MODE Solutions in the mode list after solving for the waveguide modes. The bend losses can also be found using MODE Solutions using the bent mode solver.
The coupling loss can be calculated using an FDTD simulation of the coupling region. For a discussion of different methods for doing this, see this KX post.
Note that all three of these losses would be taken into account by an FDTD simulation of the full ring resonator, but it would be difficult to figure out the magnitude of each type of loss from the total loss.
Scattering loss due to sidewall roughness is difficult to simulate due to the size of the features involved. It would be easiest to calculate it analytically (see this KX post).
Let me know if you have any more questions.